Assembly Question: Indirect Reference (looping through code)

Sam Schweighart

Is it possible to do an indirect reference in assembly? (I apologize if I'm using the wrong terminology)

I have a number of variables (100) starting at var1. Is it possible to create a loop in assembly, and add 1 to all of them?

Or do I have to name each variable and explicitly add one to each of them.

:loop
add    (some reference), #1
goto loop

.
.
.
res var 100

VS

add    var1, #1
add    var2, #1
add    var3, #1

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.
.


var1 res 1
var2 res 1
var3 res 1
.
.
.

]

Post Edited (Sam Schweighart) : 5/30/2010 3:30:15 PM GMT

Mike Green

There is no built-in indirect addressing nor are there any index registers. The sort of thing you want to do is accomplished by doing instruction modification where you calculate the address, then put it directly into an instruction to be executed. Here's one example:

   movd   addIt, #var1          ' initialize address
   mov     count, #10         ' number of items to increment
addIt:
   add   0-0,#1           ' note 0-0 is used as placeholder
   add   addIt,dest1          ' increment destination address
   djnz   count, #addIt          ' decrement count, branch if not done
'...
dest1   long   1 << 9          ' destination address starts at bit 9
count   res   1
var1   res   10

Sam Schweighart

Ahh, That makes sense. Thanks for your help!

Sam Schweighart

Actually, I have a quick question...

The place holder '0-0' ... why did you use '0-0'? Was it arbitrary? Could I just use '0' or '123' ?

Thanks!

Kye

Yeah, it just makes the code more readable.

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Nyamekye,

Mike Green

"0-0" is unlikely to be used for any other purpose and it has the value zero. It's relatively easy to spot in code.

Phil Pilgrim (PhiPi)

Sam,

When you modify an instruction to do indirect referencing, be sure to execute at least one instruction between the modifying and modified instructions (e.g. mov count, #10 and djnz count, #addIt in Mike's example). This is necessary because of the Propeller's instruction pipelining. You can use a nop if you have to.

-Phil